Highly efficient photocatalytic hydrogen evolution and simultaneous formaldehyde degradation over Z-scheme ZnIn2S4-NiO/BiVO4 hierarchical heterojunction under visible light irradiation

In this work, we successfully constructed a highly efficient Z-scheme ZnIn2S4-NiO/BiVO4 photocatalytic system and reported a breakthrough study that has uncovered the photocatalytic reaction mechanism and pathway of hydrogen (H2) evolution and simultaneous formaldehyde (HCHO) degradation over the as-prepared Z-scheme photocatalytic system. By exploring the effects of oxygen (O2) and pH on the photocatalytic activity of the Z-scheme photocatalytic system, it was found that O2 and alkalinity are necessary for photocatalytic HCHO degradation and H2 production. O2 can boost the oxidation conversion of HCHO into formic acid (HCOOH) and restrain the HCHO polymerization reaction, therefore accelerate the formation of H2 in the reaction system. With the increase of pH value from 4 to 13, the reducibility of HCHO and the electronegativity of catalyst increase, which makes the activity of catalyst system increase. The results provide a new route for the optimization of reaction conditions for simultaneous photocatalytic H2 production and HCHO degradation. Under the optimal conditions (80% of O2, 13 of pH and 1.5 mol·L−1 of HCHO initial concentration), the best H2 production activity and HCHO degradation activity are 2724.89 μmol·h−1 and 17.00 mmol·h−1, respectively. On this basis, the possible pathway and mechanism of photocatalytic H2 production and simultaneous HCHO degradation were proposed, which shed lights on elucidating the reaction path and underlying mechanism for photocatalytic H2 production and pollutant degradation.